Human gamma delta T cells are involved in chronic inflammation and in immune response to intracellular infections. The specific localization of human gamma delta T cells in disease lesions of the intestinal epithelium and skin suggest important functional roles in immune surveillance of these sites, possibly via recognition of microbial antigens and/or damage-induced self antigens. Interestingly, the intestine is also now recognized as a site of extrathymic T cell differentiation in the mouse. The prevalence of gamma delta T cells in the chicken immune system suggests a selective advantage and a greater functional role of gamma delta T cell development and function. In this revised grant application, the chicken is presented as an animal model in which to study gamma delta T cells of extrathymic origin and their functional role in response to pathogens invading the intestinal epithelium and inflammation of the skin. First, the exquisite specificity of thymic gamma delta T cell suppression in chickens by embryonic mAb treatment, without effects on alphabeta T cells or B cells, will be used to test the hypothesis that the gamma delta T cell first line of defense develops in epithelial tissues. Extrathymic sites of gamma delta T cells will be compared. Extrathymic sites, including the intestinal epithelium, will be probed with mAb to chicken thymic stromal elements to identify shared features of thymic and extrathymic microenvironments that support gamma delta T cell development. The natural occurrence of environmental pathogens that invade the chicken intestinal epithelium (Eimeria and Salmonella) will be used to test the hypothesis that gamma delta T cells of extrathymic origin are the functionally relevant population responding to epithelial infections. Comparison of the expressed gamma delta TCR repertoires of gamma delta T cells in normal and infected chickens will provide insights as to the nature of the antigen recognized by gamma delta T cells (e.g. microbial antigens or superantigens vs. damage-induced epithelial cell antigens or superantigens). A chicken line characterized by inherited scleroderma involving gamma delta T cell infiltration will be used to examine the functional role of gamma delta T cells in skin inflammation by determining the effect of thymic gamma delta T cells suppression on disease progression, and to test the trauma signal hypothesis in the absence of a resident skin gamma delta T cell population with a homogeneous TCR. This animal model will provide significant insights into the evolution of the gamma delta TCR and the function of gamma delta T cells under pathological conditions of the intestinal epithelium and skin.